1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy and, in particular, to an alkali metal oxyhalide electrochemical cell capable of discharge at temperatures up to about 200xc2x0 C.
2. Prior Art
It is known that the useful operating temperature range of a lithium electrochemical cell, such as a lithium oxyhalide cell, can be extended beyond 180.54xc2x0 C. the melting point of lithium, and up to approximately 200xc2x0 C. Although functional, conventional lithium cells manufactured for such high temperature applications exhibit serious deficiencies. In particular, it has been discovered that certain materials used in the construction of prior art lithium cells severely limit or totally inhibit the restart capability of the cell beyond a certain state of discharge. This requires that the cell be removed from the device being powered prior to the device being restarted with another cell.
According to the present invention, it has been discovered that the restart characteristics of a primary lithium oxyhalide cell or battery, which is intended for use in elevated temperature applications, can be significantly improved through the use of certain components and materials of construction.
One embodiment of the present invention describes a high capacity anode current collector which is thermally stable in a lithium oxyhalide cell and to which the electrochemically active anode material has superior bonding strength in comparison to prior art metallic current collectors. It is well known that lithium metal softens and melts at a temperature of approximately 180.54xc2x0 C. If the lithium melts, an internal short circuit usually develops. This can result in cell venting or in a violent explosion.
As a solution, certain anode alloys have been developed to extend the thermal stability of lithium cells beyond 180.54xc2x0 C. These include a lithium-aluminum alloy in which the weight percent of lithium varies between approximately 50% and 99.9%, a lithium-magnesium alloy in which the weight percent of lithium varies between approximately 67% and 99.9%, a ternary lithium-aluminum-magnesium alloy in which the weight percent of lithium varies between approximately 50% and 99.5% and a lithium-boron alloy in which the weight percent of lithium varies between approximately 40% and 99.5%. Other anode alloys are also known, but these are the most common.
In particular, it is known that a lithium-magnesium alloy can be used to extend the temperature tolerance of a lithium anode well above 180xc2x0 C. Differential scanning calorimetry measurements indicate that this particular alloy softens and melts over a temperature range of 180xc2x0 C.to well over 230xc2x0 C. depending on the amount of magnesium present.
It has also been discovered that as the magnesium content in the anode material is increased, the alloy stiffens and becomes less pliable. This has important implications for the design of the current collector. Traditionally, the anode current collector in a lithium oxyhalide cell is made from nickel, stainless steel or nickel-plated stainless steel. To provide superior adhesion while maximizing the cell""s energy density, these current collector materials are often provided in an annealed, expanded form. When the lithium content in an alloy is higher than 95%, by weight, such materials can still be used. As the lithium content is decreased, however, the anode material stiffens and adhesion to the current collector becomes problematic. When the magnesium content in the lithium anode is increased to approximately 15%, by weight, or higher, the anode is prone to delamination from the current collector as the electrode is bent, rolled or deformed. This is undesirable since intimate contact between the anode material and the current collector is lost, resulting in increased cell polarization. Additionally, delamination is known to lead to safety problems such as cell venting or explosion if the lithium oxyhalide cell is discharged to end-of-life.
According to another embodiment of the present invention, an expanded metal screen is both pulled and annealed to provide a new current collector for a lithium cell, preferably a lithium oxyhalide cell. When the magnesium content of the lithium alloy is at least 15%, by weight, and preferably between about 23% to about 27%, bonding between the anode alloy and the current collector is increased over that known by the prior art. Furthermore, the thusly fabricated anode assembly can be repeatedly bent, rolled or deformed with no visible delamination of the alloy from the current collector. Such an anode assembly provides for reduced polarization, increased energy density and increased safety when used in a lithium oxyhalide cell of the present invention. Thus, the first object of the present invention is the provision of a high capacity anode current collector which is thermally stable and to which an electrochemically active lithium alloy having a magnesium content of about 23% to about 27%, by weight, is contacted. This anode alloy has superior bonding strength to the current collector in comparison to prior art constructions.
A further embodiment of the present invention relates to a high temperature, primary lithium oxyhalide cell in which the cell""s energy density is increased and the restart capability of the cell is greatly improved in comparison to that offered by the prior art. This is accomplished through the use of a novel separator, electrolyte and electrolyte salt combination.
As discussed earlier, prior art lithium cells are known to have serious restart deficiencies after having been discharged to a particular state. Traditionally, problems concerning restarting primary lithium cells have been identified as anode, cathode or electrolyte related phenomenon. However, according to the present invention, in lithium oxyhalide cells and, in particular, those containing a lithium-magnesium anode, the restart phenomenon is attributed solely to the type of separator used. Additionally, the cell""s energy density is increased through the use of a particular combination of separator and electrolyte.
In that light, it has also been discovered that the use of a MANNIGLASS 1200 separator in combination with a thionyl chloride depolarizer and a lithium tetrachloroaluminate salt provides a lithium oxyhalide cell with increased energy density in comparison to the use of lithium electrolyte salts such as lithium tetrachlorogallate. Thus, another embodiment of the present invention relates to a high temperature lithium oxyhalide cell having increased energy density and improved restart capability in comparison to that offered by the prior art.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.